This invention relates to semiconductor memory devices and more particularly to a programmable read-only memory and method of making.
Read-only (ROM) devices are commonly used for program storage in microcomputer systems or the like. These ROMs have been mask-programmed as in my U.S. Pat. Nos. 4,208,726 or 4,219,836, assigned to Texas Instruments, or electrically programmable (EPROMs) as in my U.S. Pat. No. 4,151,021, assigned to Texas Instruments; both such mask ROMs and EPROMs are N-channel silicon-gate MOS products. For higher speed operation bipolar programmable ROMs (PROMs) are employed; see U.S. Pat. No. 4,209,894, assigned to Texas Instruments. Electrically-erasable programmable memory devices (EEPROMs) are also available as in my U.S. Pat. No. 4,122,544, assigned to Texas Instruments.
Programmable memory devices of the ROM, EPROM or EEPROM types have been very widely used in large volume production measured in many millions of units; however, the continuing trends toward lower cost, shorter access time, higher programming speed, shorter production lead times, higher reliability and reduced operating power have presented constraints for each class of products.
Mask ROMs are low cost in high volume production of a single ROM code, but have the disadvantage of a long lead time from the specification of the code to the delivery of finished devices; this lead time can be many weeks or even months from the equipment manufacturers' standpoint. EPROMs have the advantage of being programmable by the equipment manufacturer, so lead time is virtually zero, but these devices are costly because of the quartz window needed in the package for erasing by ultraviolet light. It has not been practical to deliver EPROMs in cheaper plastic (i.e. opaque) packages because of the problem of testing when erasure is impossible, and because of materials considerations. EEPROMs avoid the use of a quartz window in the package, but require a complex manufacturing process and larger cell size, so these devices will probably have limited use until the cost can be reduced. Bipolar PROMs are excellent for higher speed requirements, but are costly due to the semiconductor processing and cell size.
Programmable cells or devices are also used in fault-tolerant memory devices; that is, memory devices have redundancy in the form of rows or columns of cells which are substituted for certain rows or columms containing faulty cells. The memory array is tested after manufacture and the programmable cells used to fix the addresses of the bad rows or columns; in operation an incoming address to the memory is compared and substitution made if a match occurs. Previously, fusable polysilicon links or laser blown links have been the favored methods of implementing the programmable devices. Polysilicon links have the disadvantage of requiring removal of the protective coating on the chip over the fusable link so that a reliable open-circuit fuse blow is obtained. Laser blowing requires elaborate equipment and controls, and must be done before protective coating.
It is therefore the principal object of this invention to provide an improved programmable cell for a memory device and method of manufacture. Another object is to provide a PROM device or programmable cell of low cost and high speed. A further object is to provide a PROM device or programmable cell which can be made by a standard semiconductor manufacturing process.